Drastic BC change from 800 t0 900 yds??

I've been shooting some 140 bergers at 3130' in my 6.5 Sherman from a 300 yd. zero out to 900 yds. Everything is within a click using JBM's trajectory charts out to 800 yds and then suddenly at 900 yds I need 5 extra clicks to be on? What the heck is going on there?? Is it possible that the bc falls off that much at a certain velocity?? I don't think so. The velocity at that range would just be dropping under 2000'. What say you guys?...Rich

Assuming this is consistent and repeatable across multiple sessions, one rational explanation is that your scope's tracking could be off in that range of adjustment --
might want to verify your scope's click values in and around the "click range" where you're seeing the weirdness...

One experiment you could do: If you have a reticle that you can use for hold-offs, you could dial up into the range where everything is "working", then use the reticle to hold off for the rest of your elevation --

So just for example (example values here) -- if your verified 800 yard dope is 18 MOA and your 900 yard calculation says 22.25 MOA -- you could dial 18.25 MOA and then use your reticle to hold "up" another 4 MOA. If that is "on", but when you dial up to 22.25 MOA you are off, you know something is up with the scope.

Make sure you're at the power your reticle is calibrated at if the reticle is SFP (and preferably, you'd want to confirm your reticle's calibration).

When I saw this post I had to reply. A few of us went out labor day to do a little shooting and verify our drop charts. I was shooting my 6.5-284 running 140 VLD's at 2910 fps. I am using JBM also and I am plugging in the Litz 140 VLD info into the calculator. I have run drops out to 900 yards and had the SAME EXACT THING HAPPEN!
With a confirmed zero at 100 yards the drops have matched EXACTLY with the JBM data until I got to 900 yards. I thought that maybe we got a bad lazer reading but the 3 other rifles that we were shooting were matching the drop charts very closely. Mater of fact my 6BR matched exactly with the JBM data at 900 yards shooting 95 VLD's.

I am not near the limit of elevation and I have confirmed the scope adjustments by shooting groups at 100 yards and dialing the scope up and measuring the distance traveled. I found out that this scope actually adjusts in IPHY instead of MOA and it is plugged into JBM and is following the JBM data perfectly until I hit 900 Yards. Maybe someone has the answer.

WOW! thanks David. At least I know now that I wasn't imagining things. I talked to Mark Kenton of Kenton Ind. (the guy that custom makes the laser turrets) and he told me that this isn't uncommon. Apparently there is something going on at this velocity that can cause a SIGNIFICANT drop in B.C. Thanks again!.....Rich

Ever since one of you guys called me on the phone the other day (I think it was elkaholic) I've been thinking about this as an independent problem.

Then another case (dmgreene).

Then it hit me, I've seen this with the 6.5mm 140 VLD myself when helping a friend with a new rifle. With us, we started at 900 where the elevation was 'perfect', then we moved in. We shot at 800, 600, 500 and 200. At those ranges, we were high by 1/2 MOA (800), 1 MOA (600), .8 MOA (500) and 1 MOA (200). Another way to look at this is if we were zeroed at all the other ranges, we would have been about 1 MOA low at 900 (just like you guys) compared to the G7 predicted trajectory.

So that's 3 convincing cases of 'fishy-ness', I'm interested.

I went back to my measured drag for this bullet. It was tested on two separate occasions and there was quite a bit of scatter in the data which isn't good. However, the data indicated a reasonable match to the G7 curve.

I then entered the bullet geometry into the JBM drag calculator (McDrag based) and compared the 'computer estimated' drag curve to the measured points and G7 curve. The computer estimated curve matched very well to the G7 curve (with the .918 form factor applied). There are not interesting inflections in either the measured curve or the predicted curve anywhere in the velocity range this bullet would see over 1000 yards. In other words, there's nothing about the drag curve (measured or predicted) that explains this extra drop at 900 yards.

But the extra drop IS there.

The next possibility is that the bullet has a minor dynamic instability, like the .30 cal 168 grain MatchKing (meaning that the bullet flies with a very small 'cone-ing' motion). Dynamic instability can set in at any speed, not just transonic. A minor dynamic instability doesn't necessarily ruin accuracy, but it can affect drag, raising the actual drag of the bullet above what it would be if the bullet were flying perfectly point forward.

If this is in fact the case, I would expect to see the excessive drop at 1000 yards (and further). Anyone have drop data for 1000 yards (in addition to the good data you have for closer ranges)? I have data for 1000, but it's not well documented, so I don't have high confidence in it (never though I'd need those chicken scratches for anything like this!)

It would be really amazing if we discover that this bullet does have a dynamic instability, given it's incredible history of success in LR Hunting and competition!

I know Eric Stecker will find this interesting. He predicted months ago, a prophecy to the effect of: "Given the accuracy of these measured G7 BC's, it may be possible to diagnose problems based on a mismatch between predicted and observed trajectory instead of the old habit of doubting the accuracy of the predicted trajectory".

I hope I'm not getting ahead of myself here, but if there is in fact a minor dynamic instability with this bullet, I'm very excited to have identified it so we can make a great bullet even better!!!

Bryan we ran drop charts and confirmed at 100, 200, 300, 400, 500, 600, 700, we had to skip 800 because of the terrain and had to go to 900 yards for our next shot. My drop charts worked out EXACTLY for all other yardages except at 900. Accuracy was very good at 900 with groups running around 6" to 7" with a hunting rig. We may get a chance to go back to this area and if we do I will try to get to spot where I can shoot at 800 yards so that I can find out exactly at what range the change is occurring. We ran out of ammo so we didn't get past the 900 yard mark. It looks like we may be able to get about 1150 yard with a little work, if we are able to get that far I will give you the results.

Bryan...it was me that called you. I was really interested to hear that someone else had the exact same problem and would like very much to see what you come up with on this. I haven't had a chance to shoot past 900 yds. yet, but definetly plan on doing so. By the way, I'm shooting a 1:8 twist at 3100+......Rich

Thanks for the replies guys.
I'm certainly interested in your results past 900.
I'll be conducting some experiments to try and determine:
A) if it's a stability issue for sure, and
B) how the design has to be altered in order to fix it.

The groups (both 95 yards and 1001 yards) were 5 shots each, and were ~1/2 MOA in size.

I also fired 5 shot groups at 600, 800, and 900 yards. All of them impacted within 1/2 MOA of predicted elevation.

So I was unable to reproduce the result we've been discussing in this thread. I'm not discounting that there may be something weird going on with other combination, just sharing that in my test, the bullet behaved exactly as predicted according to the G7 BC all the way to 1000 yards.

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